Goals

<!--- What do you want to achieve? -->
Sort documents by multiple field when using MongoDB collection find or aggregate.
e.g. {$sort: {createdAt: -1, username: 1}}

Expected Results

<!--- What did you expect to happen? -->
First sort by createdAt in descending order and then sort by username in ascending order.

Actual Results

<!--- What happened instead?
e.g. the stack trace of a crash
-->
Since Dictionary in Swift or NSDictionary in Objective-C is unordered collection, the order of {$sort: {createdAt: -1, username: 1}} is uncertain, it could not make sure the consistency of sort order.

For example, using FindOptions for finding document

let options = FindOptions(limit: 20, projection: nil, sort: ["createdAt": -1, "username": 1])
collection.find(filter: ["favorite": "MongoDB Realm"], options: options)

This result will be not consistent, because the result could be sorted by createdAt than username or by username than createdAt.

Steps for others to Reproduce

<!--- What are steps OTHERS can follow to reproduce this issue? -->

Code Sample

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Version of Realm and Tooling

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In the CONTRIBUTING guidelines, you will find a script,
which will help determining some of these versions.
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Realm framework version: 10.7.2

Xcode version: 12.4

iOS/OSX version: 14.4

Dependency manager + version: Swift Package Manager